Seals are used between inner and outer wellhead tubular members to contain internal well pressure. The inner wellhead member may be a casing hanger located in a wellhead housing and that supports a string of casing extending into the well. A seal or packoff seals between the casing hanger and the wellhead housing. Alternatively, the inner wellhead member could be a tubing hanger that supports a string of tubing extending into the well for the flow of production fluid. The tubing hanger lands in an outer wellhead member, which may be a wellhead housing, a Christmas tree, or a tubing head. A packoff or seal seals between the tubing hanger and the outer wellhead member.
A variety of seals of this nature have been employed in the prior art. Prior art seals include elastomeric and partially metal and elastomeric rings. Prior art seal rings made entirely of metal for forming metal-to-metal seals are also employed. The seals may be set by a running tool, or they may be set in response to the weight of the string of casing or tubing. One type of prior art metal-to-metal seal has inner and outer walls separated by a cylindrical slot. An energizing ring is pushed into the slot in the seal to deform the inner and outer walls apart into sealing engagement with the inner and outer wellhead members. The energizing ring is a solid wedge-shaped member. The deformation of the seal's inner and outer walls exceeds the yield strength of the material of the seal ring, making the deformation permanent.
Thermal growth between the casing or tubing and the wellhead may occur, particularly with wellheads located at the surface, rather than subsea. The well fluid flowing upward through the tubing heats the string of tubing, and to a lesser degree the surrounding casing. The temperature increase may cause the tubing hanger and/or casing hanger to move axially a slight amount relative to the outer wellhead member. During the heat up transient, the tubing hanger and/or casing hanger can also move radially due to temperature differences between components and the different rates of thermal expansion from which the component materials are constructed. If the seal has been set as a result of a wedging action where an axial displacement of energizing rings induces a radial movement of the seal against its mating surfaces, then sealing forces may be reduced if there is movement in the axial direction due to pressure or thermal effects. A reduction in axial force on the energizing ring results in a reduction in the radial inward and outward forces on the inner and outer walls of the seal ring, which may cause the seal to leak. A loss of radial loading between the seal and its mating surfaces due to thermal transients may also cause the seal to leak.
One approach taken to address this leakage problem in metal-to-metal seals has been the addition of a set of wickers to the exterior of the casing hanger with the bore of the wellhead housing remaining slick. The wickers on the casing hanger sealingly engage the inner wall of the seal after it is deformed by the energizing ring and lock the seal into place. However, with travel of the seal due to thermal and pressure effects, the seal can experience a moment that leads to leakage on the side with the slick bore.
Another approach called for the use of a set of wickers on both the bore of the wellhead housing and on the exterior of the casing hanger. The wickers sealingly engaged both the outer and inner walls of the seal after they were deformed by the energizing ring. This locked the annulus seal into place on both sides, eliminating the effect of the moment that caused the seal to slide and pivot along the slick bore surface as described above. However, seal travel due to thermal growth cycles and continued increases in production pressure still resulted in seal leakage.
A need exists for a technique that addresses the seal leakage problems described above. In particular a need exists for a technique to maintain a seal between inner and outer wellhead members experiencing changes in relative positions due to thermal affects, especially those caused by high pressure and high temperature wellbore conditions. The following technique may solve these problems.